ABSTRACT Support is requested for a Keystone Symposia meeting entitled Rare and Undiagnosed Diseases: Discovery and Models of Precision Therapy, organized by Drs. William A. Gahl and Christoph Klein. The meeting will be held in Boston, Massachusetts from March 5-8, 2017. Of the 7,000 known rare diseases, only a fraction have their molecular and mechanistic bases delineated, and many other diseases have yet to be discovered. Characterization of these disorders will reveal new biochemical pathways and cell biological processes, point to potential drug targets, and provide hope for millions of affected individuals. This meeting will address approaches and best practices in pursuing rare and undiagnosed disorders, limitations in data sharing as a barrier to new disease discovery, and examples of successful descriptions of new diseases, mechanisms, and treatments. Specific aims include: 1) fostering the development of expertise in rare diseases and the definition of their natural histories, including training the next generation of rare disease investigators; 2) developing an international network of scientists and physicians collaborating on new disease discovery by sharing phenotypic and sequence data; and 3) promoting the creation of new therapeutic approaches based upon rare and new disease mechanisms. Through this meeting, participants will become familiar with rare and undiagnosed disease programs, acquire insights into new disease mechanisms, learn about potential therapeutic targets, and establish collaborations that enhance rare disorder expertise and new disease discovery. The meeting will bring together physicians who are expert in rare disorders with scientists who know metabolic pathways and mechanisms, advancing understanding and therapy. A desired outcome of the meeting includes developing lists of clinical and basic research experts for synergistic collaborations, and creating an international organization to perpetuate the joint investigation of rare and new diseases. Relevance to NCATS: the National Center for Advancing Translational Sciences houses the Office of Rare Diseases Research, which initiated the Undiagnosed Diseases Program and which gets a lot of calls from patients without a diagnosis. The NCATS mission is translational, and rare and undiagnosed disease patients are the Institute?s most needy constituents with respect to new drug development.